The present invention relates generally to ion selective electrodes, ion selective sensors, and ion selective field effect transducers, and more specifically to a solid contact material for use in such devices.
In many settings, for example the clinical laboratory or the analytical or industrial chemical laboratory, the need for rapid analysis of the concentration of a variety of ionic species or analytes in solution exists. Conventionally, in such analyses, an ion selective electrode is brought into contact with a test solution into which a reference electrode is also immersed. The ion selective electrode and the reference electrode are connected via a voltmeter and a potentiometric determination of the activity of a particular analyte in solution is made. The activity measurement may be correlated to the concentration of the analyte in solution using reference solutions or standard solutions of known concentration.
Typically, ion selective electrodes have been fabricated according to the following designs. One conventional design consists of an internal redox couple reference electrode, an ion-selective membrane, and an internal liquid electrolyte contacting the reference electrode and the membrane. The ion selective membrane is advantageously fabricated so as to include therein a particular molecule selectively complexing the ion to be analyzed, and the electrolyte advantageously includes a relatively high concentration of the ion to be analyzed and the anion of the redox couple, for example chloride in a silver/silver chloride reference. The potentiometric determination is based upon the principle that the electromotive force detected by the voltmeter is proportional to the logarithmic concentration of analyte in solution. Standard solutions of known analyte concentrations are typically used and a calibration curve is established in such analyses, and the concentration of analyte in the test sample is determined by comparison with the calibration curve. However, such a system requires maintenance of the electrode in an environment in which the electrode is not permitted to dry out, and is not amenable to electrode significant miniaturization, for example in the fabrication of planar electrodes.
Hydrogels, that is, neutral polymeric matrices incorporating salts, have been employed in ion selective electrodes in a manner similar to that of the above-described liquid electrolyte. However, hydrogels tend to swell unacceptably during use. Such swelling may have several detrimental effects, one of which is physical disruption, ultimately rupture, of the overlaying ion selective membrane, and another of which is unstable concentration of the analyte ion within the hydrogel as the swell value changes, resulting in imprecision and inaccuracy in analysis.
Another ion selective electrode design involves a reference redox couple, for example a silver/silver chloride electrode, covered with a conventional ion selective membrane, and is known as a xe2x80x9ccoated wire electrodexe2x80x9d. Such a design would appear to be amenable to miniaturization and to planar electrode fabrication. However, in an ion analysis system, it is important that junction potentials at material and phase interfaces of the overall voltametric determination circuit be as stable and reproducible as possible, so as to assure precision and accuracy in analysis. Instability in interface junction potential of the ion selective membrane/reference redox couple interface is observed in coated wire electrodes, at which a border between a domain of electronic conductivity and ionic conductivity exists, which border is void of chemical or electronic equilibria thereacross due to the lack of an internal contacting solution. Such a border is known in the art as a xe2x80x9cblockedxe2x80x9d interface.
One method of stabilizing such a junction potential is described in pending U.S. patent application Ser. No. 07/650,347, assigned to the assignee of this application, and corresponding European Patent Publication No. 0498572 A2, published Aug. 12, 1992, both of which are incorporated herein by reference. Described is a redox couple reference contacted by an ion selective membrane including a fortiophore designed to complex an ionic species of the redox couple.
U.S. Pat. No. 4,434,249 describes the preparation of ion-transfer membranes and methods of preparing such membranes from acrylic monomers and, particularly, of copolymers of at least two acrylic monomers. Specifically described is a membrane composition having a monomeric component ( greater than 88 mol %) of a non-ionic hydrophobic material such as methyl methacrylate (MMA) and a monomeric component (3-12% mol %) of an acrylic monomer containing ionogenous groups such as methacrylamidopropyltrimethylammonium chloride (MAPTAC). The patent describes an optimum concentration level of the ionogenous monomer for transfer of ions across this bulk material, departures from which will decrease transference. No teaching is made of utilizing the material as an interface between two different conducting domains, for example, electronic and ionic domains.
European Patent Publication No. 0325562 A2, incorporated herein by reference, describes an ion selective electrode in which an ion exchange resin is incorporated between an electrochemical redox couple and an ion selective membrane, the ion exchange resin being selected so as to have ions counter to those to be analyzed chemically linked to the polymer defining the resin. The publication teaches the use of standard, commercially-available ion exchange resins, for example, Amberlite, Dowex, and Nafion, which typically have a capacity on the order of 4.3 milliequivalents per gram, that is, about 2.5xc3x971021 charged sites per gram of material. While neither aqueous swell values nor adherence characteristics of this approach are documented in the publication, ion exchange resins typically swell unacceptably in such circumstances, even to the point of dissolving, and adherence between an ion exchange resin and an inert substrate is generally thought to be poor. Indeed, adhesion between many polymeric materials incorporating charged groups and other polymeric layers or substrates must often be augmented by an intermediate, inert, mesh-like layer such as, for example, silated polyvinylchloride.
Ion selective membranes have also been used as gate materials in ion selective field effect transistors. An electronic/ionic domain boundary complication exists in that technology as well.
Therefore, it remains a challenge in the art to formulate a solid state material to serve as an interface between the electronic and ionic domains of ion selective sensors, ion selective field effect transducers, and the like, such material having a rapidly equilibrating, reproducible swell value below that at which physical disruption of any device in which it is employed occurs, which is easy to prepare and use, and which adheres well to typical substrates and to adjacent layers in devices in which it is employed.
Accordingly, it is a general purpose of the present invention to provide a means and method of establishing or maintaining a more stable equilibria between electronic and ionic domains, which equilibria facilitate stable and reproducible junction potentials in electrochemical devices, using a material which is easy to prepare and use, which adheres well to typical substrates and adjacent materials in electrochemical devices in which it is employed, and which has a stable and reproducible swell value.
The foregoing and other objects and advantages of the present invention are achieved by providing an ion selective sensor comprising an electrically conductive material providing a reference redox couple, a polymeric material having immobilized charged sites provided on the electrically conductive material, and an ion selective material covering the polymeric material. Preferably, the polymeric material carries less than about 1.63xc3x971021 immobilized charge sites per gram. The ion selective electrode may additionally comprise a substrate onto which the electrically conductive reference electrode material is deposited.
It is another object of the present invention to provide an ion selective sensor comprising an electrically conductive material providing a reference redox couple, a polymeric material provided on the electrically conductive material, the polymeric material having immobilized sites of charge opposite that of mobile ions involved in the redox couple, and an ion selective material covering the polymeric material.
It is another object of the present invention to provide a method of providing a stable and reproducible electrical interface between a solid contact reference providing a redox couple and an ion selective material of an ion selective sensor or the like, comprising contacting both the solid contact reference and the ion selective material with a polymeric material having immobilized charged sites, the polymeric material carrying less than about 1.63xc3x971021 immobilized charged sites per gram.
It is another object of the present invention to provide a method of forming an ion selective sensor comprising the steps of providing an electrically conductive material creating a redox couple including a portion of the electrically conductive material, coating the redox couple with a polymeric material having immobilized charged sites, and covering the polymeric material with an ion selective material.
It is another object of the present invention to provide a method of providing a stable and reproducible electrical interface between a solid contact reference providing a redox couple and an ion selective material of an ion selective sensor or the like, comprising contacting both the solid contact reference and the ion selective material with a polymeric material having immobilized sites of charge opposite that of mobile ions involved in the redox couple.
It is another object of the present invention to provide a method of forming an ion selective sensor comprising the steps of providing an electrically conductive material, creating a redox couple including a portion of the electrically conductive material, coating the redox couple with a polymeric material having immobilized sites of charge opposite that of mobile ions involved in the redox couple, and covering the polymeric material with an ion selective material.
It is another object of the present invention to provide a chemical responsive field effect transducer comprising a layer of semiconductor material having a surface, a pair of first diffusion regions of a first doping characteristic at the surface, a second diffusion region of a second doping characteristic at the surface between the pair of first diffusion regions, an electrically insulating material provided on the surface, a polymeric material having immobilized charged sites provided on the electrically conductive material, the polymeric material carrying less than about 1.63xc3x971021 immobilized charged sites per gram, the polymeric material separated from the diffusion regions by the insulating material, an ion selective material provided on the polymeric material, and an electrically insulating encapsulant sealing the transducer from a sample region while leaving exposed a region of the ion selective material for exposure to a sample.
It is still another object of the present invention to provide a method of making a chemical responsive field effect transducer comprising the steps of providing a layer of semiconductor material having a surface, doping a pair of first regions at the surface to provide a first doping characteristic therein, doping a second region of said surface between the pair of first regions so as to provide a second doping characteristic therein, covering the diffusion regions with an electrically insulating material, providing a polymeric material adjacent the electrically insulating material, the polymeric material having immobilized charged sites and carrying less than about 1.63xc3x971021 immobilized charged sites per gram, covering the polymeric material with an ion selective material, and encapsulating the transducer with an electrically insulating encapsulant so as to separate the transducer from a sample region while leaving exposed a region of the ion selective material for exposure to a sample.
These and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of elements set forth in the specification and covered by the claims appended hereto.